The invention relates to devices and methods for reducing surface friction drag on the hull of a vessel traveling through water.
Surface friction drag or “skin friction” drag is a significant component of the total power required to propel a surface vessel through water. Reducing surface friction drag enables vessels to travel at higher speeds and/or more efficiently. Accordingly, reducing surface friction drag has been the subject of a great deal of research in the field of vessel hull design for both surface and submerged vessels.
The magnitude of surface friction drag on a particular vessel hull depends, in part, upon the viscosity of the liquid through which the hull is traveling (usually fresh or salt water), the density of the liquid and the surface tension between the liquid and the submerged surface of the hull.
As shown schematically in FIG. 2, the effects of surface friction drag are focused in a “boundary layer” 22, a layer of liquid in which momentum is transferred from the surface 16 of the hull 14 to the liquid 12. Momentum transfer is the greatest in the portion of the liquid that is closest to the surface 16 of the hull 14 and decreases to the edge 26 of the boundary layer 22. Momentum transfer in the boundary layer 22 results in a reduction in the velocity of the water 12 relative to the surface 16 of the hull 14, as well as turbulence. A velocity gradient 24 shows the decrease in relative velocity of the water 12 from the edge of the boundary layer 26 to the surface 16 of the hull 14. Relative velocity is represented by the length of each arrow.
One means of reducing surface friction drag is the introduction of a gas into the boundary layer 22, which reduces the fluid density and viscosity in the boundary layer 22. The relatively low density and viscosity of the gas results in less momentum transfer, and therefore, less surface friction drag. This technique is sometimes referred to in the art as “air lubrication.”
Air lubrication has been successfully implemented in hovercraft, in which the vessel sits atop a large cushion of air. Air cushions are not practical for use with vessels having a significant draft, however, because water pressure increases with depth, which causes the air cushion to quickly rise to the surface of the water. Enormous amounts of power are required to push an air cushion down into a few inches of water. This problem has been addressed, in part, by using small bubbles of air (i.e., micro-bubbles) instead of a larger air cushion. Small bubbles rise much more slowly in water than a large air cushion.
Full-scale use of micro-bubbles has been proven very difficult. The inventions of the prior art have faced three major technical challenges in successful use of micro-bubbles to reduce surface friction: (1) injecting micro-bubbles at a sufficient volumetric rate to fill a significant portion of the boundary layer, (2) keeping the micro-bubbles from migrating out of the boundary layer, and (3) adjusting the volumetric flow rate of micro-bubbles as the velocity of the vessel changes.
Most prior art air lubrication systems use either a pump or pressurized air to supply the volume of micro-bubbles. This approach is deficient in several respects. Firstly, power must be expended to pump or pressurize the air. In all cases, the power expended to pump or pressurize the air completely offsets the power savings from reduced surface friction drag. Secondly, it is very difficult to inject pumped or pressurized air into the boundary layer. A typical boundary layer is only a few millimeters thick near the bow of the vessel, which is where the air is injected in most prior art systems. Given that the micro-bubbles themselves are at least one millimeter in diameter and are typically injected at an angle to the direction of flow F of the boundary layer, it is very difficult to prevent the micro-bubbles from passing through the boundary layer and into the free-flow water area. Thirdly, the prior art does not provide for an injection flow rate for micro-bubbles that varies in proportion to the vessel's speed. This results in the micro-bubble injection rate being ideal at only one speed. At all other speeds, the injection rate is higher or lower than the ideal rate.
Other prior art air lubrication systems, such as the system described in U.S. Pat. No. 6,125,781, purport to aerate water flow into the boundary layer of a vessel hull using a tube that has one or more ports on the submerged surface of the vessel hull and is open to the air at the opposite end. In such prior art systems, it is hypothesized that air will be “sucked” through the port(s) and into the boundary layer. This hypothesis is based on flawed assumptions. It has been determined that these types of systems only work on vessels with very shallow drafts, traveling at high speeds. For example, air would not begin to be sucked into the boundary layer along the hull of a vessel having a draft of 3.973 inches until the vessel reached a speed 90.6 miles per hour. This is not a feasible speed for most surface vessels.
Another prior art system for reducing surface drag is referred to in the art as a ventilated step chine, which is used primarily in high-performance watercraft. An example of a ventilated step chine design is described in U.S. Pat. No. 5,452,676. Although ventilated step chines appear to provide some performance and efficiency improvements, the ventilated step chines of the prior art do not entrain significant amounts of air into the boundary layer. This is due, in part, to the fact that the ventilated step chines of the prior art do not produce turbulent mixing of air and water in the vicinity of the step. Conventional ventilated step chines merely reduce the effective surface area of the hull, so that the frictional effects of water act on a smaller area. The reduction is only a small percentage of the total surface area of the hull; therefore, ventilated step chines provide little, if any, surface friction reduction.
Accordingly, there is a need for an efficient air lubrication system that is capable of entraining air bubbles into the boundary layer of a wide variety of vessels, including those having a substantial draft and a system that functions at much more reasonable speeds.